Permanent Porosity in the Room-Temperature Magnet and Magnonic Material V(TCNE)2

Materials that simultaneously exhibit permanent porosity and high-temperature magnetic order could lead to advances in fundamental physics and numerous emerging technologies. Herein, we show that the archetypal molecule-based magnet and magnonic material V(TCNE)₂ (TCNE = tetracyanoethylene) can be desolvated to generate a room-temperature microporous magnet. The solution-phase reaction of V(CO)₆ with TCNE yields V(TCNE)₂·0.95CH₂Cl₂, for which a characteristic temperature of T* = 646 K is estimated from a Bloch fit to variable-temperature magnetization data. Removal of the solvent under reduced pressure affords the activated compound V(TCNE)₂, which exhibits a T* value of 590 K and permanent microporosity (Langmuir surface area of 850 m²/g). The porous structure of V(TCNE)₂ is accessible to the small gas molecules H₂, N₂, O₂, CO₂, ethane, and ethylene. While V(TCNE)₂ exhibits thermally activated electron transfer with O₂, all the other studied gases engage in physisorption. The T* value of V(TCNE)₂ is slightly modulated upon adsorption of H₂ (T* = 583 K) or CO₂ (T* = 596 K), while it decreases more significantly upon ethylene insertion (T* = 459 K). These results provide an initial demonstration of microporosity in a room-temperature magnet and highlight the possibility of further incorporation of small-molecule guests, potentially even molecular qubits, toward future applications.

Metal-organic frameworks as O2-selective adsorbents for air separations

Oxygen is a critical gas in numerous industries and is produced globally on a gigatonne scale, primarily through energy-intensive cryogenic distillation of air. The realization of large-scale adsorption-based air separations could enable a significant reduction in associated worldwide energy consumption and would constitute an important component of broader efforts to combat climate change. Certain small-scale air separations are carried out using N2-selective adsorbents, although the low capacities, poor selectivities, and high regeneration energies associated with these materials limit the extent of their usage. In contrast, the realization of O2-selective adsorbents may facilitate more widespread adoption of adsorptive air separations, which could enable the decentralization of O2 production and utilization and advance new uses for O2. Here, we present a detailed evaluation of the potential of metal-organic frameworks (MOFs) to serve as O2-selective adsorbents for air separations. Drawing insights from biological and molecular systems that selectively bind O2, we survey the field of O2-selective MOFs, highlighting progress and identifying promising areas for future exploration. As a guide for further research, the importance of moving beyond the traditional evaluation of O2 adsorption enthalpy, ΔH, is emphasized, and the free energy of O2 adsorption, ΔG, is discussed as the key metric for understanding and predicting MOF performance under practical conditions. Based on a proof-of-concept assessment of O2 binding carried out for eight different MOFs using experimentally derived capacities and thermodynamic parameters, we identify two existing materials and one proposed framework with nearly optimal ΔG values for operation under user-defined conditions. While enhancements are still needed in other material properties, the insights from the assessments herein serve as a guide for future materials design and evaluation. Computational approaches based on density functional theory with periodic boundary conditions are also discussed as complementary to experimental efforts, and new predictions enable identification of additional promising MOF systems for investigation.

Semiquinone radical-bridged M2 (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me6tren)2MII 2(C6H4O2 2-)]2+ (Me6tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me6tren)2MII 2(C6H4O2 -˙)]3+. Single-crystal X-ray diffraction shows that the Me6tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p-benzosemiquinone-transition metal magnetic exchange coupling for the first time and find that the nickel(ii) complex exhibits a substantial J < -600 cm-1, resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal-semiquinone exchange constants of J = -144(1) and -252(2) cm-1, respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm-1, respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear NiII complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation.

Enhancing catalytic alkane hydroxylation by tuning the outer coordination sphere in a heme-containing metal-organic framework

Catalytic heme active sites of enzymes are sequestered by the protein superstructure and are regulated by precisely defined outer coordination spheres. Here, we emulate these protective functions in the porphyrinic metal-organic framework PCN-224 by post-synthetic acetylation and subsequent hydroxylation of the Zr6 nodes. A suite of physical methods demonstrates that both transformations preserve framework structure, crystallinity, and porosity without modifying the inner coordination spheres of the iron sites. Single-crystal X-ray analyses establish that acetylation replaces the mixture of formate, benzoate, aqua, and terminal hydroxo ligands at the Zr6 nodes with acetate ligands, and hydroxylation affords nodes with seven-coordinate, hydroxo-terminated Zr4+ ions. The chemical influence of these reactions is probed with heme-catalyzed cyclohexane hydroxylation as a model reaction. By virtue of passivated reactive sites at the Zr6 nodes, the acetylated framework oxidizes cyclohexane with a yield of 68(8)%, 2.6-fold higher than in the hydroxylated framework, and an alcohol/ketone ratio of 5.6(3).

Insensitivity of Magnetic Coupling to Ligand Substitution in a Series of Tetraoxolene Radical-Bridged Fe2 Complexes

The elucidation of magnetostructural correlations between bridging ligand substitution and strength of magnetic coupling is essential to the development of high-temperature molecule-based magnetic materials. Toward this end, we report the series of tetraoxolene-bridged Fe^II₂ complexes [(Me₃TPyA)₂Fe₂(^RL)]ⁿ⁺ (Me₃TPyA = tris(6-methyl-2-pyridylmethyl)amine; n = 2: ^OMeLH₂ = 3,6-dimethoxy-2,5-dihydroxo-1,4-benzoquinone, ^ClLH₂ = 3,6-dichloro-2,5-dihydroxo-1,4-benzoquinone, Na₂[^NO₂L] = sodium 3,6-dinitro-2,5-dihydroxo-1,4-benzoquinone; n = 4: ^SMe₂L = 3,6-bis(dimethylsulfonium)-2,5-dihydroxo-1,4-benzoquinone diylide) and their one-electron-reduced analogues. Variable-temperature dc magnetic susceptibility data reveal the presence of weak ferromagnetic superexchange between Fe^II centers in the oxidized species, with exchange constants of J = +1.2(2) (R = OMe, Cl) and +0.3(1) (R = NO₂, SMe₂) cm^-1. In contrast, X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy establish a ligand-centered radical in the reduced complexes. Magnetic measurements for the radical-bridged species reveal the presence of strong antiferromagnetic metal-radical coupling, with J = -57(10), -60(7), -58(6), and -65(8) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. The minimal effects of substituents in the 3- and 6-positions of ^RL^x-• on the magnetic coupling strength is understood through electronic structure calculations, which show negligible spin density on the substituents and associated C atoms of the ring. Finally, the radical-bridged complexes are single-molecule magnets, with relaxation barriers of U_eff = 50(1), 41(1), 38(1), and 33(1) cm^-1 for R = OMe, Cl, NO₂, and SMe₂, respectively. Taken together, these results provide the first examination of how bridging ligand substitution influences magnetic coupling in semiquinoid-bridged compounds, and they establish design criteria for the synthesis of semiquinoid-based molecules and materials.

Metal-Organic Framework Magnets

Metal-organic frameworks represent the ultimate chemical platform on which to develop a new generation of designer magnets. In contrast to the inorganic solids that have dominated permanent magnet technology for decades, metal-organic frameworks offer numerous advantages, most notably the nearly infinite chemical space through which to synthesize predesigned and tunable structures with controllable properties. Moreover, the presence of a rigid, crystalline structure based on organic linkers enables the potential for permanent porosity and postsynthetic chemical modification of the inorganic and organic components. Despite these attributes, the realization of metal-organic magnets with high ordering temperatures represents a formidable challenge, owing largely to the typically weak magnetic exchange coupling mediated through organic linkers. Nevertheless, recent years have seen a number of exciting advances involving frameworks based on a wide range of metal ions and organic linkers. This review provides a survey of structurally characterized metal-organic frameworks that have been shown to exhibit magnetic order. Section 1 outlines the need for new magnets and the potential role of metal-organic frameworks toward that end, and it briefly introduces the classes of magnets and the experimental methods used to characterize them. Section 2 describes early milestones and key advances in metal-organic magnet research that laid the foundation for structurally characterized metal-organic framework magnets. Sections 3 and 4 then outline the literature of metal-organic framework magnets based on diamagnetic and radical organic linkers, respectively. Finally, Section 5 concludes with some potential strategies for increasing the ordering temperatures of metal-organic framework magnets while maintaining structural integrity and additional function.

A Dimeric Hydride-Bridged Complex with Geometrically Distinct Iron Centers Giving Rise to an S = 3 Ground State

Structural and spectroscopic characterization of the dimeric iron hydride complex [Ph₂B(^tBuIm)₂FeH]₂ reveals an unusual structure in which a tetrahedral iron(II) site (S = 2) is connected to a square planar iron(II) site (S = 1) by two bridging hydride ligands. Magnetic susceptibility reveals strong ferromagnetic coupling between iron centers, with a coupling constant of J = +110(12) cm^-1, to give an S = 3 ground state. High-frequency and -field electron paramagnetic resonance (HFEPR) spectroscopy confirms this model. A qualitative molecular orbital analysis of the electronic structure, as supported by electronic structure calculations, reveals that the observed spin configuration results from the orthogonal alignment of two geometrically distinct four-coordinate iron fragments held together by highly covalent hydride ligands.

Thiosemiquinoid Radical-Bridged CrIII2 Complexes with Strong Magnetic Exchange Coupling

Semiquinoid radical bridging ligands are capable of mediating exceptionally strong magnetic coupling between spin centers, a requirement for the design of high-temperature magnetic materials. We demonstrate the ability of sulfur donors to provide much stronger coupling relative to their oxygen congeners in a series of dinuclear complexes. Employing a series of chalcogen donor-based bis(bidentate) benzoquinoid bridging ligands, the series of complexes [(TPyA)₂Cr₂(^RL^4-)]²⁺ (^OLH₄ = 1,2,4,5-tetrahydroxybenzene, ^OSLH₄ = 1,2-dithio-4,5-dihydroxybenzene, ^SLH₄ = 1,2,4,5-tetrathiobenzene, TPyA = tris(2-pyridylmethyl)amine) was synthesized. Variable-temperature dc magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange coupling between Cr^III centers in these complexes, with exchange constants of J = -2.83(3) (^OL^4-), -2.28(5) (^OSL^4-), and -1.80(2) (^SL^4-) cm^-1. Guided by cyclic voltammetry and spectroelectrochemical measurements, chemical one-electron oxidation of these complexes gives the radical-bridged species [(TPyA)₂Cr₂(^RL^3-•)]³⁺. Variable-temperature dc susceptibility measurements in these complexes reveal the presence of strong antiferromagnetic metal-semiquinoid radical coupling, with exchange constants of J = -352(10) (^OL^3-•), - 401(8) (^OSL^3-•), and -487(8) (^SL^3-•) cm^-1. These results provide the first measurement of magnetic coupling between metal ions and a thiosemiquinoid radical, and they demonstrate the value of moving from O to S donors in radical-bridged metal ions in the design of magnetic molecules and materials.

Selective Binding and Quantitation of Calcium with a Cobalt-Based Magnetic Resonance Probe

We report a cobalt-based paramagnetic chemical exchange saturation transfer (PARACEST) magnetic resonance (MR) probe that is able to selectively bind and quantitate the concentration of Ca²⁺ ions under physiological conditions. The parent LCo complex features CEST-active carboxamide groups and an uncoordinated crown ether moiety in close proximity to a high-spin pseudo-octahedral Co^II center. Addition of Na⁺, Mg²⁺, K⁺, and Ca²⁺ leads to binding of these metal ions within the crown ether. Single-crystal X-ray diffraction and solid-state magnetic measurements reveal the presence of a cation-specific coordination environment and magnetic anisotropy of Co^II, with axial zero-field splitting parameters for the Na⁺- and Ca²⁺-bound complexes differing by over 90%. Owing to these differences, solution-based measurements under physiological conditions indicate reversible binding of Na⁺ and Ca²⁺ to give well-separated CEST peaks at 69 and 80 ppm for [LCoNa]⁺ and [LCoCa]²⁺, respectively. Dissociation constants for different cation-bound complexes of LCo, as determined by ¹H NMR spectroscopy, demonstrate high selectivity toward Ca²⁺. This finding, in conjunction with the large excess of Na⁺ in physiological environments, minimizes interference from related cations, such as Mg²⁺ and K⁺. Finally, variable-[Ca²⁺] CEST spectra establish the ratio between the CEST peak intensities for the Ca²⁺- and Na⁺-bound probes (CEST_80 ppm/CEST_69 ppm) as a measure of [Ca²⁺], providing the first example of a ratiometric quantitation of Ca²⁺ concentration using PARACEST. Taken together, these results demonstrate the ability of transition metal PARACEST probes to afford a concentration-independent measure of [Ca²⁺] and provide a new approach for designing MR probes for cation sensing.

Reversible redox switching of magnetic order and electrical conductivity in a 2D manganese benzoquinoid framework

We report a 2D manganese benzoquinoid network that undergoes simultaneous redox switching of magnetic order and electrical conductivity.

Materials with switchable magnetic and electrical properties may enable future spintronic technologies, and thus hold the potential to revolutionize how information is processed and stored. While reversible switching of magnetic order or electrical conductivity has been independently realized in materials, the ability to simultaneously switch both properties in a single material presents a formidable challenge. Here, we report the 2D manganese benzoquinoid framework (Me₄N)₂[MnII2(L^2–)₃] (H₂L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone), as synthesized via post-synthetic counterion exchange. This material is paramagnetic above 1.8 K and exhibits an ambient-temperature electrical conductivity of σ_{295 K} = 1.14(3) × 10^–13 S cm^–1 (E_a = 0.74(3) eV). Upon soaking in a solution of sodium naphthalenide and 1,2-dihydroacenaphthylene, this compound undergoes a single-crystal-to-single-crystal (SC–SC) reduction to give Na₃(Me₄N)₂[Mn₂L₃]. Structural and spectroscopic analyses confirm this reduction to be ligand-based, and as such the anionic framework is formulated as [MnII2(L^3–˙)₃]^5–. Magnetic measurements confirm that this reduced material is a permanent magnet below T_c = 41 K and exhibits a conductivity value of σ_{295 K} = 2.27(1) × 10^–8 S cm^–1 (E_a = 0.489(8) eV), representing a remarkable 200 000-fold increase over the parent material. Finally, soaking the reduced compound in a solution of [Cp₂Fe]⁺ affords Na(Me₄N)[MnII2(L^2–)₃] via a SC–SC process, with magnetic and electrical properties similar to those observed for the original oxidized material. Taken together, these results highlight the ability of metal benzoquinoid frameworks to undergo reversible, simultaneous redox switching of magnetic order and electrical conductivity.

A Series of Early Lanthanide Chloranilate Frameworks with a Square Grid Topology

A series of lanthanide chloranilate frameworks containing a (4,4)-net with LaIII, CeIII, NdIII, SmIII, and EuIII have been synthesised and structurally characterised. Two structure types of square grids were obtained for these frameworks. Type 1 consists of the formula (Et4N)[Ln(can)2(H2O)] (Ln=LaIII, CeIII, NdIII; H2can=chloranilic acid) and crystallised in the tetragonal space group I4/m, featuring a nine-coordinate lanthanide ion with a coordinated water molecule and four chloranilate ligands. Type 2, (Et4N)[Ln(can)2] (SmIII and EuIII) crystallised in the I4/mcm space group, and contains an eight-coordinate lanthanide ion without a coordinated water molecule. A single-crystal-to-single-crystal transformation was carried out for (Et4N)[Nd(can)2(H2O)] on removal of the coordinated aqua ligand.

Dramatic enhancement in pH sensitivity and signal intensity through ligand modification of a dicobalt PARACEST probe

A ratiometric dicobalt PARACEST pH probe that exhibits dramatic enhancements in sensitivity and signal intensity over analogous probes is reported.

A Ferric Semiquinoid Single-Chain Magnet via Thermally-Switchable Metal-Ligand Electron Transfer

We report the synthesis of a semiquinoid-bridged single-chain magnet, as generated through a thermally induced metal-ligand electron transfer. Reaction of FeCl₃ with 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH₂) in the presence of (NMe₄)Cl gave the compound (NMe₄)₂[LFeCl₂]. Together, variable-temperature X-ray diffraction, Mössbauer spectra, Raman spectra, and dc magnetic susceptibility reveal a transition from a chain containing (L^2-)Fe^II units to one with (L^3-•)Fe^III upon decreasing temperature, with a transition temperature of T_1/2 = 213 K. The dc magnetic susceptibility measurements show strong metal-radical coupling within the chain, with a coupling constant of J = -81 cm^-1, and ac susceptibility data reveal slow magnetic relaxation, with a relaxation barrier of Δ_τ = 55(1) cm^-1. To our knowledge, this compound provides the first example of a semiquinoid-bridged single-chain magnet.

Effect of Magnetic Coupling on Water Proton Relaxivity in a Series of Transition Metal GdIII Complexes

A fundamental challenge in the design of bioresponsive (or bioactivated) Gd^III-based magnetic resonance (MR) imaging probes is the considerable background signal present in the "preactivated" state that arises from outer-sphere relaxation processes. When sufficient concentrations of a bioresponsive agent are present (i.e., a detectable signal in the image), the inner- and outer-sphere contributions to r₁ may be misinterpreted to conclude that the agent has been activated, when it has not. Of the several parameters that determine the observed MR signal of an agent, only the electron relaxation time ( T_1e) impacts both the inner- and outer-sphere relaxation. Therefore, strategies to minimize this background signal must be developed to create a near zero-background (or truly "off" state) of the agent. Here, we demonstrate that intramolecular magnetic exchange coupling when Gd^III is coupled to a paramagnetic transition metal provides a means to overcome the contribution of second- and outer-sphere contributions to the observed relaxivity. We have prepared a series of complexes with the general formula LMLn(μ-O₂CCH₃)(O₂CCH₃)₂ (M = Co, Cu, Zn). Solid-state magnetic susceptibility measurements reveal significant magnetic coupling between Gd^III and the transition metal ion. Nuclear magnetic relaxation dispersion (NMRD) analysis confirms that the observed differences in relaxivity are associated with the modulation of T_1e at Gd^III. These results clearly demonstrate that magnetic exchange coupling between Gd^III and a transition metal ion can provide a significant decrease in T_1e (and therefore the relaxivity of Gd^III). This design strategy is being exploited to prepare new generations of preclinical bioresponsive MR imaging probes with near zero-background.

High-throughput synthesis and characterization of nanocrystalline porphyrinic zirconium metal-organic frameworks

We describe and employ a high-throughput screening method to accelerate the synthesis and identification of pure-phase, nanocrystalline metal-organic frameworks (MOFs). We demonstrate the efficacy of this method through its application to a series of porphyrinic zirconium MOFs, resulting in the isolation of MOF-525, MOF-545, and PCN-223 on the nanoscale.

A structurally-characterized peroxomanganese(iv) porphyrin from reversible O2 binding within a metal-organic framework

Within a MOF, a side-on peroxomanganese(iv) porphyrin has been isolated and comprehensively examined.

The role of peroxometal species as reactive intermediates in myriad biological processes has motivated the synthesis and study of analogous molecular model complexes. Peroxomanganese(iv) porphyrin complexes are of particular interest, owing to their potential ability to form from reversible O₂ binding, yet have been exceedingly difficult to isolate and characterize in molecular form. Alternatively, immobilization of metalloporphyrin sites within a metal–organic framework (MOF) can enable the study of interactions between low-coordinate metal centers and gaseous substrates, without interference from bimolecular reactions and axial ligation by solvent molecules. Here, we employ this approach to isolate the first rigorously four-coordinate manganese(ii) porphyrin complex and examine its reactivity with O₂ using infrared spectroscopy, single-crystal X-ray diffraction, EPR spectroscopy, and O₂ adsorption analysis. X-ray diffraction experiments reveal for the first time a peroxomanganese(iv) porphyrin species, which exhibits a side-on, η² binding mode. Infrared and EPR spectroscopic data confirm the formulation of a peroxomanganese(iv) electronic structure, and show that O₂ binding is reversible at ambient temperature, in contrast to what has been observed in molecular form. Finally, O₂ gas adsorption measurements are employed to quantify the enthalpy of O₂ binding as h_ads = –49.6(8) kJ mol^–1. This enthalpy is considerably higher than in the corresponding Fe- and Co-based MOFs, and is found to increase with increasing reductive capacity of the M^II/III redox couple.

Ratiometric quantitation of redox status with a molecular Fe2 magnetic resonance probe

We demonstrate the ability of a molecular Fe2 complex to enable magnetic resonance (MR)-based ratiometric quantitation of redox status, namely through redox-dependent paramagnetic chemical exchange saturation transfer (PARACEST). Metalation of a tetra(carboxamide) ligand with FeII and/or FeIII in the presence of etidronate ion affords analogous FeII2, FeIIFeIII, and FeIII2 complexes. Both FeII2 and FeIIFeIII complexes give highly-shifted, sharp, and non-overlapping NMR spectra, with multiple resonances for each complex corresponding to exchangeable carboxamide protons. These protons can be selectively irradiated to give CEST peaks at 74 and 83 ppm vs. H2O for the FeIIFeIII complex and at 29, 40 and 68 ppm for the FeII2 complex. The CEST spectra obtained from a series of samples containing mixtures of FeII2 and FeIIFeIII are correlated with independently-determined open-circuit potentials to construct a Nernstian calibration curve of potential vs. CEST peak intensity ratio. In addition, averaged intensities of phantom images collected on a 9.4 T MRI scanner show analogous Nernstian behavior. Finally, both the FeII2 and FeIIFeIII forms of the complex are stable to millimolar concentrations of H2PO4-/HPO42-, CO32-, SO42-, CH3COO-, and Ca2+ ions, and the FeIII2 form is air-stable in aqueous buffer and shows >80% viability in melanoma cells at millimolar concentration. The stability suggests the possible application of this or related complexes for in vivo studies. To our knowledge, this concentration-independent method based on a single Fe2 probe provides the first example of MR-based ratiometric quantitation of redox environment.

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(Im^R)₃}FeN], R = ^tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(Im^R)₃}Fe(NR')]⁺, R = Mes, R' = 1-adamantyl (3), ^tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMEN^Ar)FeN]⁺ (TIMEN^Ar = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by ⁵⁷Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3d_z² (a₁) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

Spin-crossover and high-spin iron(ii) complexes as chemical shift 19F magnetic resonance thermometers

The potential utility of paramagnetic transition metal complexes as chemical shift 19F magnetic resonance (MR) thermometers is demonstrated. Further, spin-crossover FeII complexes are shown to provide much higher temperature sensitivity than do the high-spin analogues, owing to the variation of spin state with temperature in the former complexes. This approach is illustrated through a series of FeII complexes supported by symmetrically and asymmetrically substituted 1,4,7-triazacyclononane ligand scaffolds bearing 3-fluoro-2-picolyl derivatives as pendent groups (L x ). Variable-temperature magnetic susceptibility measurements, in conjunction with UV-vis and NMR data, show thermally-induced spin-crossover for [Fe(L1)]2+ in H2O, with T1/2 = 52(1) °C. Conversely, [Fe(L2)]2+ remains high-spin in the temperature range 4-61 °C. Variable-temperature 19F NMR spectra reveal the chemical shifts of the complexes to exhibit a linear temperature dependence, with the two peaks of the spin-crossover complex providing temperature sensitivities of +0.52(1) and +0.45(1) ppm per °C in H2O. These values represent more than two-fold higher sensitivity than that afforded by the high-spin analogue, and ca. 40-fold higher sensitivity than diamagnetic perfluorocarbon-based thermometers. Finally, these complexes exhibit excellent stability in a physiological environment, as evidenced by 19F NMR spectra collected in fetal bovine serum.

pH-Dependent spin state population and 19F NMR chemical shift via remote ligand protonation in an iron(ii) complex

An Fe^II complex that features a pH-dependent spin state population and ¹⁹F chemical shift, by virtue of a variable ligand protonation state, is described.

Dioxygen binding at a four-coordinate cobaltous porphyrin site in a metal–organic framework: structural, EPR, and O2 adsorption analysis

The binding of O₂ at a four-coordinate cobaltous porphyrin site within a metal–organic framework is examined through single-crystal X-ray diffraction, EPR spectroscopy, and O₂ adsorption measurements.

An S = 12 semiquinoid radical-bridged Mn6wheel complex assembled from an asymmetric redox-active bridging ligand

The asymmetric redox-active ligand 4,5-bis(pyridine-2-carboxamido)-1,2-catechol (^N,OLH₄) is synthesized and metalated to afford theS= 12 semiquinoid radical-bridged wheel complex [Mn^II₆(^N,OL³⁻˙)₆]⁶⁻.

A 2D Semiquinone Radical-Containing Microporous Magnet with Solvent-Induced Switching from Tc = 26 to 80 K

The incorporation of tetraoxolene radical bridging ligands into a microporous magnetic solid is demonstrated. Metalation of the redox-active bridging ligand 2,5-dichloro-3,6-dihydroxy-1,4-benzoquinone (LH2) with Fe(II) affords the solid (Me2NH2)2[Fe2L3]·2H2O·6DMF. Analysis of X-ray diffraction, Raman spectra, and Mössbauer spectra confirm the presence of Fe(III) centers with mixed-valence ligands of the form (L3)(8-) that result from a spontaneous electron transfer from Fe(II) to L(2-). Upon removal of DMF and H2O solvent molecules, the compound undergoes a slight structural distortion to give the desolvated phase (Me2NH2)2[Fe2L3], and a fit to N2 adsorption data of this activated compound gives a BET surface area of 885(105) m(2)/g. Dc magnetic susceptibility measurements reveal a spontaneous magnetization below 80 and 26 K for the solvated and the activated solids, respectively, with magnetic hysteresis up to 60 and 20 K. These results highlight the ability of redox-active tetraoxolene ligands to support the formation of a microporous magnet and provide the first example of a structurally characterized extended solid that contains tetraoxolene radical ligands.

A series of tetraazalene radical-bridged M2 (M = CrIII, MnII, FeII, CoII) complexes with strong magnetic exchange coupling

The ability of tetraazalene radical bridging ligands to mediate exceptionally strong magnetic exchange coupling across a range of transition metal complexes is demonstrated. The redox-active bridging ligand N,N',N'',N'''-tetra(2-methylphenyl)-2,5-diamino-1,4-diiminobenzoquinone (NMePhLH2) was metalated to give the series of dinuclear complexes [(TPyA)2M2(NMePhL2-)]2+ (TPyA = tris(2-pyridylmethyl)amine, M = MnII, FeII, CoII). Variable-temperature dc magnetic susceptibility data for these complexes reveal the presence of weak superexchange interactions between metal centers, and fits to the data provide coupling constants of J = -1.64(1) and -2.16(2) cm-1 for M = MnII and FeII, respectively. One-electron reduction of the complexes affords the reduced analogues [(TPyA)2M2(NMePhL3-˙)]+. Following a slightly different synthetic procedure, the related complex [(TPyA)2CrIII2(NMePhL3-˙)]3+ was obtained. X-ray diffraction, cyclic voltammetry, and Mössbauer spectroscopy indicate the presence of radical NMePhL3-˙ bridging ligands in these complexes. Variable-temperature dc magnetic susceptibility data of the radical-bridged species reveal the presence of strong magnetic interactions between metal centers and ligand radicals, with simulations to data providing exchange constants of J = -626(7), -157(7), -307(9), and -396(16) cm-1 for M = CrIII, MnII, FeII, and CoII, respectively. Moreover, the strength of magnetic exchange in the radical-bridged complexes increases linearly with decreasing M-L bond distance in the oxidized analogues. Finally, ac magnetic susceptibility measurements reveal that [(TPyA)2Fe2(NMePhL3-˙)]+ behaves as a single-molecule magnet with a relaxation barrier of Ueff = 52(1) cm-1. These results highlight the ability of redox-active tetraazalene bridging ligands to enable dramatic enhancement of magnetic exchange coupling upon redox chemistry and provide a rare opportunity to examine metal-radical coupling trends across a transmetallic series of complexes.

Spin crossover iron(ii) complexes as PARACEST MRI thermometers

We demonstrate the potential utility of spin crossover iron(ii) complexes as temperature-responsive paramagnetic chemical exchange saturation transfer (PARACEST) contrast agents in magnetic resonance imaging (MRI) thermometry.

An azophenine radical-bridged Fe2 single-molecule magnet with record magnetic exchange coupling

One-electron reduction of the complex [(TPyA)2Fe(II)2((NPh)L(2-))](2+) (TPyA = tris(2-pyridylmethyl)amine, (NPh)LH2 = azophenine = N,N',N",N'''-tetraphenyl-2,5-diamino-1,4-diiminobenzoquinone) affords the complex [(TPyA)2Fe(II)2((NPh)L(3-•))](+). X-ray diffraction and Mössbauer spectroscopy confirm that the reduction occurs on (NPh)L(2-) to give an S = 1/2 radical bridging ligand. Dc magnetic susceptibility measurements demonstrate the presence of extremely strong direct antiferromagnetic exchange between S = 2 Fe(II) centers and (NPh)L(3-•) in the reduced complex, giving an S = 7/2 ground state with an estimated coupling constant magnitude of |J| ≥ 900 cm(-1). Mössbauer spectroscopy and ac magnetic susceptibility reveal that this complex behaves as a single-molecule magnet with a spin relaxation barrier of U(eff) = 50(1) cm(-1). To our knowledge, this complex exhibits by far the strongest magnetic exchange coupling ever to be observed in a single-molecule magnet.

Trigonal Mn3 and Co3 clusters supported by weak-field ligands: a structural, spectroscopic, magnetic, and computational investigation into the correlation of molecular and electronic structure

Transamination of divalent transition metal starting materials (M(2)(N(SiMe(3))(2))(4), M = Mn, Co) with hexadentate ligand platforms (R)LH(6) ((R)LH(6) = MeC(CH(2)NPh-o-NR)(3) where R = H, Ph, Mes (Mes = Mesityl)) or (H,Cy)LH(6) = 1,3,5-C(6)H(9)(NHPh-o-NH(2))(3) with added pyridine or tertiary phosphine coligands afforded trinuclear complexes of the type ((R)L)Mn(3)(py)(3) and ((R)L)Co(3)(PMe(2)R')(3) (R' = Me, Ph). While the sterically less encumbered ligand varieties, (H)L or (Ph)L, give rise to local square-pyramidal geometries at each of the bound metal atoms, with four anilides forming an equatorial plane and an exogenous pyridine or phosphine in the apical site, the mesityl-substituted ligand ((Mes)L) engenders local tetrahedral coordination. Both the neutral Mn(3) and Co(3) clusters feature S = (1)/(2) ground states, as determined by direct current (dc) magnetometry, (1)H NMR spectroscopy, and low-temperature electron paramagnetic resonance (EPR) spectroscopy. Within the Mn(3) clusters, the long internuclear Mn-Mn separations suggest minimal direct metal-metal orbital overlap. Accordingly, fits to variable-temperature magnetic susceptibility data reveal the presence of weak antiferromagnetic superexchange interactions through the bridging anilide ligands with exchange couplings ranging from J = -16.8 to -42 cm(-1). Conversely, the short Co-Co interatomic distances suggest a significant degree of direct metal-metal orbital overlap, akin to the related Fe(3) clusters. With the Co(3) series, the S = (1)/(2) ground state can be attributed to population of a single molecular orbital manifold that arises from mixing of the metal- and o-phenylenediamide (OPDA) ligand-based frontier orbitals. Chemical oxidation of the neutral Co(3) clusters affords diamagnetic cationic clusters of the type [((R)L)Co(3)(PMe(2)R)(3)](+). Density functional theory (DFT) calculations on the neutral (S = (1)/(2)) and cationic (S = 0) Co(3) clusters reveal that oxidation occurs at an orbital with contributions from both the Co3 core and OPDA subunits. The predicted bond elongations within the ligand OPDA units are corroborated by the ligand bond perturbations observed by X-ray crystallography.

Multi-site reactivity: reduction of six equivalents of nitrite to give an Fe6(NO)6 cluster with a dramatically expanded octahedral core

Reaction of NO(2)(-) with the octahedral cluster ((H)L)(2)Fe(6) in the presence of a proton source affords the hexanitrosyl cluster ((H)L)(2)Fe(6)(NO)(6). This species forms via a proton-induced reduction of six nitrite molecules per cluster, utilizing each site available on the polynuclear core. Formation of the hexanitrosyl cluster is accompanied by a near 2-fold expansion of the ((H)L)(2)Fe(6) core volume, where intracore Fe-Fe interactions are overcome by strong π-bonding between Fe centers and NO ligands. A core volume of this magnitude is rare in octahedral metal clusters not supported by interstitial atoms. Moreover, the structural flexibility afforded by the ((H)L)(2)Fe(6) platform highlights the potential for other reaction chemistry involving species with metal-ligand multiple bonds. Carrying out the reaction of the cluster [((H)L)(2)Fe(6)(NCMe)(6)](4+) with nitrite in the absence of a proton source serves to forestall the nitrite reduction and enables clean isolation of the intermediate hexanitro cluster [((H)L)(2)Fe(6)(NO(2))(6)](2-).

Slow magnetic relaxation and electron delocalization in an S = 9/2 iron(II∕III) complex with two crystallographically inequivalent iron sites

The magnetic, electronic, and Mössbauer spectral properties of [Fe(2)L(μ-OAc)(2)]ClO(4), 1, where L is the dianion of the tetraimino-diphenolate macrocyclic ligand, H(2)L, indicate that 1 is a class III mixed valence iron(II∕III) complex with an electron that is fully delocalized between two crystallographically inequivalent iron sites to yield a [Fe(2)](V) cationic configuration with a S(t) = 9∕2 ground state. Fits of the dc magnetic susceptibility between 2 and 300 K and of the isofield variable-temperature magnetization of 1 yield an isotropic magnetic exchange parameter, J, of -32(2) cm(-1) for an electron transfer parameter, B, of 950 cm(-1), a zero-field uniaxial D(9∕2) parameter of -0.9(1) cm(-1), and g = 1.95(5). In agreement with the presence of uniaxial magnetic anisotropy, ac susceptibility measurements reveal that 1 is a single-molecule magnet at low temperature with a single molecule magnetic effective relaxation barrier, U(eff), of 9.8 cm(-1). At 5.25 K the Mössbauer spectra of 1 exhibit two spectral components, assigned to the two crystallographically inequivalent iron sites with a static effective hyperfine field; as the temperature increases from 7 to 310 K, the spectra exhibit increasingly rapid relaxation of the hyperfine field on the iron-57 Larmor precession time of 5 × 10(-8) s. A fit of the temperature dependence of the average effective hyperfine field yields |D(9∕2)| = 0.9 cm(-1). An Arrhenius plot of the logarithm of the relaxation frequency between 5 and 85 K yields a relaxation barrier of 17 cm(-1).